ARQ retransmission with reordering scheme employing multiple redundancy versions and receiver/transmitter therefor

ABSTRACT

An ARQ retransmission method in a communication system, wherein data packets comprising modulation symbols are retransmitted based on an automatic repeat request and subsequently combined with previously received data packets, the symbols of said data packets being modulated by a mapping unit employing a predetermined signal constellation. The retransmitted data packets being retransmitted in form of a selected one of a plurality of different redundancy versions. According to the invention, the bits to be transmitted are reordered prior to modulation over the retransmissions in accordance with the selected redundancy version.

[0001] The present invention relates to an ARQ retransmission method ina communication system. Further, the invention concerns a respectivereceiver and a transmitter.

[0002] A common technique in communication systems with unreliable andtime-varying channel conditions is to correct errors based on automaticrepeat request (ARQ) schemes together with a forward error correction(FEC) technique called hybrid ARQ (HARQ). If an error is detected by acommonly used cyclic redundancy check (CRC), the receiver of thecommunication system requests the transmitter to send additionalinformation (data packets retransmission) to improve the probability ofcorrectly decoding the erroneous packet.

[0003] A packet will be encoded with the FEC before transmission.Depending on the content of the retransmission and the way the bits arecombined with previously transmitted information, S. Kallel, Analysis ofa type II hybrid ARQ scheme with code combining, IEEE Transactions onCommunications, Vol.38, No. 8, August 1990 and S. Kallel, R. Link, S.Bakhtiyari, Throughput performance of Memory ARQ schemes, IEEETransactions on Vehicular Technology, Vol.48, No. 3, May 1999 definethree different types of ARQ schemes:

[0004] Type I: The erroneous received packets are discarded and a newcopy of the same packet is retransmitted and decoded separately. Thereis no combining of earlier and later received versions of that packet.

[0005] Type II: The erroneous received packets are not discarded, butare combined with additional retransmissions for subsequent decoding.Retransmitted packets sometimes have higher coding rates (coding gain)and are combined at the receiver with the stored soft-information fromprevious transmissions.

[0006] Type III: Is the same as Type II with the constraint eachretransmitted packet is now self-decodable. This implies that thetransmitted packet is decodable without the combination with previouspackets. This is useful if some packets are damaged in such a way thatalmost no information is reusable. If all transmissions carry identifieddata, this can be seen as a special case called HARQ Type III with asingle redundancy version.

[0007] HARQ Type II and III schemes are obviously more intelligent andshow a performance gain with respect to Type I, because they provide theability to reuse information from of previously received erroneouspackets. There exist basically three schemes of reusing the redundancyof previously transmitted packets:

[0008] Soft-Combining

[0009] Code-Combining

[0010] Combination of Soft- and Code-Combining

[0011] Soft-Combining

[0012] Employing soft-combining the retransmission packets carryidentical information compared with the previously received information.In this case the multiple received packets are combined either by asymbol-by-symbol or by a bit-by-bit basis as for example disclosed in D.Chase, Code combining: A maximum-likelihood decoding approach forcombining an arbitrary number of noisy packets, IEEE Trans. Commun.,Vol. COM-33, pp. 385-393, May 1985 or B. A. Harvey and S. Wicker, PacketCombining Systems based on the Viterbi Decoder, IEEE Transactions onCommunications, Vol. 42, No. 2/3/4, April 1994.

[0013] In case of employing symbol-level combining, the retransmittedpackets have to carry identical modulation symbols to the previouslytransmitted erroneous packets. In this case the multiple receivedpackets are combined at modulation symbol level. A common technique isthe maximum ratio combining (MRC), also called average diversitycombining (ADC), of the multiple received symbols, where after Ntransmissions the sum/average of the matching symbols is buffered.

[0014] In case of employing bit-level combining the retransmittedpackets have to carry identical bits to the previously transmittederroneous packets. Here, the multiple received packets are combined atbit level after demodulation. The bits can be either mapped in the sameway onto the modulation symbols as in previous transmissions of the samepacket or can be mapped differently. In case the mapping is the same asin previous transmissions also symbol-level combining can be applied. Acommon combining technique is the addition of calculated log-likelihoodratios (LLRs), especially if using so-called Turbo Codes for the FEC asknown for example from C. Berrou, A. Glavieux, and P. Thitimajshima,Near Shannon Limit Error-Correcting Coding and Decoding: Turbo-Codes,Proc. ICC '93, Geneva, Switzerland, pp. 1064-1070, May 1993; S. Le Goff,A. Glavieux, C. Berrou, Turbo-Codes and High Spectral EfficiencyModulation, IEEE SUPERCOMM/ICC '94, Vol. 2 , pp. 645-649, 1994; and A.Burr, Modulation and Coding for Wireless Communications, PearsonEducation, Prentice Hall, ISBN 0-201-39857-5, 2001. Here, after Ntransmissions the sum of the LLRs of the matching bits is buffered.

[0015] Code-Combining

[0016] Code-combining concatenates the received packets, in order togenerate a new code word (decreasing code rate with increasing number oftransmission). Hence, the decoder has to be aware of how to combine thetransmissions at each retransmission instant in order to perform acorrect decoding (code rate depends on retransmissions). Code-combiningoffers a higher flexibility with respect to soft-combining, since thelength of the retransmitted packets can be altered to adapt to channelconditions. However, this requires more signaling data to be transmittedwith respect to soft-combining.

[0017] Combination of Soft- and Code-Combining

[0018] In case the retransmitted packets carry some symbols/bitsidentical to previously transmitted symbols/bits and somecode-symbols/bits different from these ones, the identicalcode-symbols/bits are combined using soft-combing as described in thesection titled “Soft-Combining” while the remaining code-symbols/bitswill be combined using code-combining. Here, the signaling requirementswill be similar to code-combining.

[0019] It has been shown in M. P. Schmitt, Hybrid ARQ Scheme employingTCM and Packet Combining, Electronics Letters Vol. 34, No. 18, September1998 that HARQ performance for Trellis Coded Modulation (TCM) can beenhanced by rearranging the symbol constellation for theretransmissions. There, the performance gain results from the maximizingthe Euclidean distances between the mapped symbols over theretransmissions, because the rearrangement has been performed on asymbol basis. Considering high-order modulation schemes (with modulationsymbols carrying more than two bits) the combining methods employingsoft-combining have a major drawback: The bit reliabilities withinsoft-combined symbols will be in a constant ratio over allretransmissions, i.e. bits which have been less reliable from previousreceived transmissions will still be less reliable after having receivedfurther transmissions and, analogous, bits which have been more reliablefrom previous received transmissions will still be more reliable afterhaving received further transmissions. Generally, HARQ schemes do nottake into account the variations in bit-reliabilities. These variationsdowngrade the decoder performance significantly. Mainly, the variationsresult from two reasons.

[0020] First, the varying bit reliabilities evolve from the constraintof two-dimensional signal constellation mapping, where modulationschemes carrying more than 2 bits per symbol cannot have the same meanreliabilities for all bits under the assumption that all symbols aretransmitted equally likely. The term mean reliabilities is consequentlymeant as the reliability of a particular bit over all symbols of asignal constellation.

[0021] Employing a signal constellation for a 16 QAM modulation schemeaccording to FIG. 1 showing a Gray encoded signal constellation with agiven bit-mapping order i₁q₁i₂q₂, the bits mapped onto the symbolsdiffer significantly from each other in mean reliability in the firsttransmission of the packet. In more detail, bits i₁ and q₁ have a highmean reliability, as these bits are mapped to half spaces of the signalconstellation diagram with the consequences that their reliability isindependent from the fact of whether the bit transmits a one or a zero.

[0022] In contrast thereto, bits i₂ and q₂ have a low mean reliability,as their reliability depends on the fact of whether they transmit a oneor a zero. For example, for bit i₂, ones are mapped to outer columns,whereas zeros are mapped to inner columns. Similarly, for bit q₂, onesare mapped to outer rows, whereas zeros are mapped to inner rows.

[0023] For the second and each further retransmissions the bitreliabilities will stay in a constant ratio to each other, which isdefined by the signal constellation employed in the first transmission,i.e. bits i₁ and q₁ will always have a higher mean reliability than bitsi₂ and q₂ after any number of retransmissions.

[0024] Second, employing partly soft-combining, suppose that alltransmitted bits would have identical reliability after the firsttransmission. Even then variations in bit reliabilities would beintroduced over retransmissions, because reliabilities for those bitswhich are retransmitted (and soft-combined) would increase, whereasreliabilities of not retransmitted bits would stay unchanged. Moreover,bits which are not transmitted in the first transmission and thentransmitted in retransmissions (transmitting additional redundancy)emphasize this effect.

[0025] In co-pending PCT/EP01/01982 a method has been suggested that inorder to enhance the decoder performance, it would be quite beneficialto have equal or near to equal mean bit reliabilities after eachreceived transmission of a packet. Hence, the bit reliabilities aretailored over the retransmissions in a way that the mean bitreliabilities get averaged aged out. This is achieved by choosing apredetermined first and at least second signal constellation for thetransmissions, such that the combined mean bit reliabilities for therespective bits of all transmissions are nearly equal. I.e. bits whichhave been highly reliable in the first transmission are mapped in such away that they become less reliable in the second transmission and viceversa.

[0026] Hence, the signal constellation rearrangement results in achanged bit mapping, wherein the Euclidean distances between themodulation symbols can be altered from retransmission to retransmissiondue to the movement of the constellation points. As a result, the meanbit reliabilities can be manipulated in a desired manner and averagedout to increase the performance the FEC decoder at the receiver.

[0027] In the solution proposed above, the benefits of the constellationrearrangement are realized for the concept of the HARQ TYPE II/IIIsingle redundancy version schemes.

[0028] The object of the present invention is to provide an ARQretransmission method and transmitter, which effectively avoidsdowngrading of the decoder performance caused by the variations in bitreliabilities.

[0029] The object is solved by a method, transmitter and receiver as setforth in the independent claims.

[0030] The invention is based on the recognition that the conventionalschemes do not consider this specific content (set of bits) of eachtransmission for reordering the bits. Hence, in order to obtain aperformance gain, the reordering has to be done depending on the contentof each transmitted redundancy version. Consequently, the invention canbe seen as providing an ARQ Type-II/III scheme using multiple redundancyversions under consideration of the content of the transmittedredundancy version. This results in a significant gain in the decoderperformance.

[0031] For a better understanding of the invention, preferredembodiments, which will be described in the following with reference tothe accompanying drawings show:

[0032]FIG. 1: an exemplary constellation illustrating a 16 QAMmodulation scheme with Gray-encoded bit symbols,

[0033]FIGS. 2a and 2 b: two examples for signal constellations for a 16QAM modulation scheme with Gray-encoded bit symbols,

[0034]FIG. 3: a generated bit sequence from a rate 1/3 FEC encoder,

[0035]FIG. 4: a chosen sequence for a rate 1/2 transmission systemgenerated from the sequence shown in FIG. 3 with an indication of thebit reliabilities,

[0036]FIG. 5: a bit sequence for the second transmission, wherein thebits are shifted by two to the right,

[0037]FIG. 6: a bit sequence for the second transmission, wherein thebit positions are switched using different mappers.

[0038]FIG. 7: a bit sequence for the first transmission redundancyversion 1 and a first pair of mapper/interleaver,

[0039]FIG. 8: a bit sequence for the second transmission for aredundancy version 2 with the same mapper/interleaver as for the firsttransmission,

[0040]FIG. 9: a bit sequence for the second transmission a redundancyversion 2 with different mappers/interleavers as for the firsttransmission,

[0041]FIG. 10: resulting bit sequences from possible combinations ofredundancy versions and mappers/interleavers,

[0042]FIG. 11: a first embodiment of a communication system in which themethod of the present invention is carried out,

[0043]FIG. 12: a second embodiment of a communication system in whichthe method of the present invention is carried out,

[0044]FIG. 13: a diagram indicating the performance of severalconventional strategies versus the strategy according to the method ofthe invention.

[0045] In the following the concept of a Log-Likelihood-Ratio (LLR) willbe described as a metric for the bit reliabilities. First the straightforward calculation of the bit LLRs within the mapped symbols for asingle transmission will be shown. Then the LLR calculation will beextended to the multiple transmission case.

[0046] Single Transmission

[0047] The mean LLR of the i-th bit b_(n) ^(i) under the constraint thatsymbol s_(n) has been transmitted for a transmission over a channel withadditive white gaussian noise (AWGN) and equally likely symbols yields$\begin{matrix}{{{{LLR}_{b_{n}^{i}|r_{n}}\left( r_{n} \right)} = {{\log\left\lbrack {\sum\limits_{({{m|b_{m}^{i}} = b_{n}^{i}})}^{\quad}\quad ^{{- \frac{E_{s}}{N_{0}}} \cdot d_{n,m}^{2}}} \right\rbrack} - {\log\left\lbrack {\sum\limits_{({m|{b_{m}^{i} \neq b_{n}^{i}}})}^{\quad}\quad ^{{- \frac{E_{s}}{N_{0}}} \cdot d_{n,m}^{2}}} \right\rbrack}}},} & (1)\end{matrix}$

[0048] where r_(n)=s_(n) denotes the mean received symbol under theconstraint the symbol s_(n) has been transmitted (AWGN case), d_(n,m) ²denotes the square of the Euclidean distance between the received symbolr_(n) and the symbol s_(m),and E_(s)/N_(o) denotes the observedsignal-to-noise ratio.

[0049] It can be seen from Equation (1) that the LLR depends on thesignal-to-noise ratio E_(s)/N_(o) and the Euclidean distances d_(n,m)between the signal constellation points.

[0050] Multiple Transmissions

[0051] Considering multiple transmissions the mean LLR after the k-thtransmission of the i-th bit b_(n) ^(i) under the constraint thatsymbols s_(n) ^((j)) have been transmitted over independent AWGNchannels and equally likely symbols yields $\begin{matrix}{{{{LLR}_{b_{n}^{i}|{\bigcap_{j = 1}^{k}r_{n}^{(j)}}}\left( {r_{n}^{(1)},r_{n}^{(2)},\ldots \quad,r_{n}^{(k)}} \right)} = {{\log\left\lbrack {\sum\limits_{({{m|b_{m}^{i}} = b_{n}^{i}})}^{\quad}\quad ^{- {\sum\limits_{j = 1}^{k}\quad {{(\frac{E_{s}}{N_{0}})}^{(j)} \cdot {(d_{n,m}^{(j)})}^{2}}}}} \right\rbrack} - {\log\left\lbrack {\sum\limits_{({m|{b_{m}^{i} \neq b_{n}^{i}}})}^{\quad}\quad ^{- {\sum\limits_{j = 1}^{k}\quad {{(\frac{E_{s}}{N_{0}})}^{(j)} \cdot {(d_{n,m}^{(j)})}^{2}}}}} \right\rbrack}}},} & (2)\end{matrix}$

[0052] where j denotes the j-th transmission ((j−1)-th retransmission).Analogous to the single transmission case the mean LLRs depend on thesignal-to-noise ratios and the Euclidean distances at each transmissiontime.

[0053] It is clear to a skilled person, that an approximation of theLLRs can be obtained by a simplified calculation to the above detailedequations.

[0054] In the following, the case of a 16-QAM system will be exemplarilyconsidered resulting in 2 high reliable and 2 low reliable bits, wherefor the low reliable bits the reliability depends on transmitting a oneor a zero (see FIG. 1). Hence, overall there exist 2 levels ofreliabilities wherein the second level can be further subdivided.

[0055] Level 1 (High Reliability, 2 bits): Bit mapping for ones (zeros)separated into the positive (negative) real half space for the i-bitsand the imaginary half space the q-bits. Here, there is no differencewhether the ones are mapped to the positive or to the negative halfspace.

[0056] Level 2 (Low Reliability, 2 bits): Ones (zeros) are mapped toinner (outer) columns for the i-bits or to inner (outer) rows for theq-bits. Since there is a difference for the LLR depending on the mappingto the inner (outer) columns and rows, Level 2 is further classified:

[0057] Level 2a: Mapping of i_(n) to inner columns and q_(n) to innerrows respectively.

[0058] Level 2b: Inverted mapping of Level 2a: Mapping of i_(n) to outercolumns and q_(n) to outer rows respectively.

[0059] To ensure an optimal averaging process over the transmissions forall bits the levels of reliabilities have to be altered.

[0060] It has to be considered that the bit-mapping order is open priorinitial transmission, but has to remain through retransmissions, e.g.bit-mapping for initial transmission: i₁q₁i₂q₂

bit-mapping all retransmissions: i₁q₁i₂q₂.

[0061] Some examples for possible constellations are shown in FIG. 2.The resulting bit reliabilities according to FIG. 2 are given inTable 1. TABLE 1 Constellation bit i₁ bit q₁ bit i₂ bit q₂ 1 HighReliability High Reliability Low Reliability Low Reliability (Level 1)(Level 1) (Level 2b) (Level 2b) 2 Low Reliability Low Reliability HighReliability High Reliability (Level 2a) (Level 2a) (Level 1) (Level 1) 3Low Reliability Low Reliability High Reliability High Reliability (Level2b) (Level 2b) (Level 1) (Level 1) 4 High Reliability High ReliabilityLow Reliability Low Reliability (Level 1) (Level 1) (Level 2a) (Level2a)

[0062] In the following, it is assumed that m denotes the retransmissionnumber parameter, with m=0 denoting the first transmission of a packetin the ARQ context. Further let b denote the number of bits that form asymbol in the mapping entity. Typically, b can be any integer number,where the most often used values for communication systems are aninteger power of 2.

[0063] Without loss of generality it can be further assumed that thenumber of bits n that are used as input to the interleaving process isdividable by b, i.e. n is an integer multiple of b. Those skilled in theart will perceive that if this should not be the case, then the sequenceof input bits can be easily appended by dummy bits until the abovecondition is met.

[0064] In the following an example of a simple Gray-mapped 16-QAMtransmission scheme with FEC rate 1/2 (S_(n): systematic bits-P_(n):parity bits), which is generated from a systematic encoder of rate 1/3(see FIG. 3) by puncturing will be considered. A sequence and orderingof bits as shown in FIG. 4 could be selected for the 1^(st) transmission(TX). FIG. 4 shows the generated sequence of FIG. 3 with an indicationof the bit reliabilities.

[0065] A simple conventional HARQ Type-III scheme with a singleredundancy version would transmit in all requested retransmissions theidentical sequence (having the identical mapping M₁ or identicalinterleaving I₁). The 1^(st) transmission is usually not interleaved,however also not to interleave can be viewed as having an interleaverwith equal input and output streams. This results after combining allreceived (and requested) transmissions in large variations of bitreliabilities. E.g. S₁ and P₁ would be highly reliable (transmitted ntimes with high reliability) whereas S₂ and P₄ would be less reliable(transmitted n times with low reliability). As stated earlier, this willdowngrade decoding performance at the receiver.

[0066] The performance of this basic scheme can be increased byswitching the reliabilities for required retransmissions to average outthe reliabilities for all transmitted bits. This can be achieved by anumber of different specific implementations, where 2 possible solutionsare depicted below in FIG. 5 and FIG. 6. This technique can beimplemented either by interleaving the bits differently than in the1^(st) transmission or by using different mapping rules for themodulation symbols. In the following this will be denoted as using a2^(nd) mapper M₂ or a 2^(nd) interleaver I₂.

[0067]FIG. 5 shows a bit sequence for the 2^(nd) transmission, wherein,in order to average bit reliabilities, the bits are shifted by 2 to theright using different interleavers for transmission

[0068]FIG. 6 shows a bit sequence for the 2^(nd) transmission, wherein,in order to average bit reliabilities, the bit positions are switchedusing different mappers for transmissions.

[0069] In case of using just 2 different mappers (M_(n)) or interleavers(I_(n)) all successive transmissions are then mapped or interleaved suchthat no mapper/interleaver is used 2 times more often than the otherone, e.g.: TABLE 2 TX Strategy 1 Strategy 2 1 I₁/M₁ I₁/M₁ 2 I₂/M₂ I₂/M₂3 I₁/M₁ I₂/M₂ 4 I₂/M₂ I₁/M₁ 5 I₁/M₁ I₁/M₁ 6 I₂/M₂ I₂/M₂ 7 I₁/M₁ I₂/M₂ .. . . . . . . .

[0070] It should be noted that for 16-QAM the usage of 4 differentmappers provides a better performance and just using 2 mappers providesa sub optimum solution. 2 mappers are chosen to keep the example simple.

[0071] It can be seen from the table above the performances of thestrategy 1 and 2 are equal or similar, hence, it does not make adifference if choosing mapper/interleaver M₁/I₁, or M₂/I₂ for the 3^(rd)TX (transmission). For the 4^(th) TX, however, it has to be taken careto choose the complementary mapper/interleaver with respect to the3^(rd) TX.

[0072] A simple prior art HARQ Type-III scheme with multiple redundancyversions would retransmit the systematic bits in the 2^(nd) TX plus theadditional parity bits, which have not been transmitted in the first TX.For simplicity the example is chosen such that the number of bits pertransmissions is kept constant and exactly 2 transmissions can carry allencoded bits (systematic and parity). To guarantee self-decodableretransmissions all systematic bits are retransmitted. It will howeverbe appreciated by those skilled in the art, that also non-self decodableretransmissions can be used to carry out the invention. FIG. 7 shows abit sequence for the 1^(st) TX as RV₁ & M₁ ¹/I₁ ¹.

[0073] For conventional schemes with multiple redundancy versions—nottaking the variations in bit reliabilities into account, i.e. having asingle mapper/interleaver as shown in the bit sequence for the sequencefor the 2^(nd) transmission RV₂ & M₁ ²/I₁ ² in FIG. 8—a similar problemarises as for schemes with a single redundancy version. Low reliablesystematic bits from the 1^(st) TX will be low reliable in the 2^(nd)transmission.

[0074] Using 2 mappers/interleavers (see FIG. 9) the averaging will beperformed for the systematic bits. However, after 2 transmissionsaveraging of the reliabilities is only possible for the bits transmittedtwice so far (in this example the systematic bits). In the 3^(rd) TX oneis free of choice which redundancy version to transmit RV₁, or RV₂(performance for both possibilities should be very similar).

[0075] The example described above having 2 redundancy versions (RV₁ andRV₂) basically provides 4 combinations of redundancy versions andmappers/interleavers (see Table 3 and FIG. 10): TABLE 3 PossibleCombinations RV₁ & I₁ ¹/M₁ ¹ RV₁ & I₂ ¹/M₂ ¹ RV₂ & I₁ ²/M₁ ² RV₂ & I₂²/M₂ ²

[0076] In the following the set of bits transmitted in the 1^(st) TXwill be labeled RV₁ (redundancy version 1) and the set of bitstransmitted in the 2^(nd) TX will be labeled RV₂. Also, themappers/interleavers are linked to the redundancy versions by asuperscript. In the shown example the interleaver pattern and mappingfor I_(n) ¹/M_(n) ¹ and I_(n) ²/M_(n) ² (n=1, 2) are equal, which is aspecial case, because the positions of the systematic and parity bitsare aligned to each other in both redundancy versions.

[0077] In accordance with the present invention, the mapper/interleaverhas to be selected according to the chosen redundancy version in orderto average out the reliabilities of the systematic and parity bits. Thisis contrary to the single redundancy version case, whereby the thirdtransmission one can select any mapper/interleaver.

[0078] In the following, a strategy for selecting the mapper/interleaverdepending on the transmitted redundancy version in order to average outall bit reliabilities is proposed.

[0079] 1^(st) TX

[0080] Let us assume the combinations RV₁ & I₁ ¹/M₁ ¹ is selected forthe 1^(st) TX—any other combination could also be selected for 1^(st)transmission (assuming equal/similar performance considering a singletransmission).

[0081] 2^(nd) TX

[0082] In the 2^(nd) TX the remaining redundancy version should betransmitted (in this case RV₂), where the reliabilities for all bitswhich have been already transmitted in the 1^(st) TX (in this case allsystematic bits) have to be averaged, i.e. low reliable systematic bitshave to be high reliable now. This is achieved by transmitting RV₂ withI₂ ²/M₂ ².

[0083] 3^(rd) TX

[0084] For the 3^(rd) TX one is free which redundancy version totransmit, however it has to be combined with a mapper/interleaver, whichhas not been yet chosen for this redundancy version, i.e. RV₁ & I₂ ¹/M₁¹ in strategy 1 and RV& I₁ ²/M₁ ² in strategy 2. This ensures theaveraging of the parity bits, which are transmitted in the current setof bits.

[0085] 4^(th) TX

[0086] For the 4^(th) TX the combination, which is left over has to beselected. This guarantees the averaging of the remaining set of paritybits and makes sure to transmit the set of parity bits, which have justbeen transmitted once so far.

[0087] 5^(th) and Further TX

[0088] After the 4^(th) TX the averaging process is finished. Hencethere is a free choice of redundancy version and mapper/interleavercombination. For following TXs the rules applied to TXs 1-4 have to beconsidered. TABLE 4 TX Strategy 1 Strategy 2 1 RV₁ & RV₁ & I₁ ¹/M₁ ¹ I₁¹/M₁ ¹ 2 RV₂ & RV₂ & I₂ ²/M₂ ² I₂ ²/M₂ ² 3 RV₁ & RV₂ & I₁ ²/M₁ ² I₂ ¹/M₂¹ 4 RV₂ & RV₁ & I₂ ¹/M₂ ¹ I₁ ²/M₁ ² 5 . . . . . .

[0089] In the provided example the positions of the systematic bits forboth redundancy version RV₁ and RV₂ (considering samemapper/interleaver) are equal (see FIG. 10). This is not generally thecase (especially for different coding rates) and is clearly asimplification. The shown example is intended to show the generalprocedure, which can be easily extended to more general cases mentionedbelow.

[0090] The proposed method is not restricted to 2 redundancy versions.Instead it can be extended to any number N of redundancy versions, whichare selected to be transmitted consecutively and repeated after Ntransmissions as in a general HARQ Type II/III scheme with N redundancyversions.

[0091] Under the assumption that m denotes the actual mapper/interleaverversion (m=1 . . . M) the number of mappers/interleavers per redundancyversion might be any integer number M (resulting in at most into N-Mdifferent mappers/interleavers, where N denotes the total number ofredundancy versions and M the number of mappers/interleavers perredundancy version), where the mapping rules or interleaver patterns arenot necessarily designed to perform a perfect averaging ofreliabilities. According to the example in Table 4, the general methodis shown in Table 5, where (as mentioned earlier) all I_(m) ^(n)/M_(m)^(n) might have different mapping rules or interleaver patterns. TABLE 5TX Combination 1 RV₁ & I₁ ¹/M₁ ¹ 2 RV₂ & I₁ ²/M₁ ² 3 RV₃ & I₁ ³/M₁ ³ . .. . . . N RV_(N) & I₁ ^(N)/M₁ ^(N) N + 1 RV₁ & I₂ ¹/M₂ ¹ . . . . . . 2NRV_(N) & I₂ ^(N)/M₂ ^(N) . . . . . . N · (M − 1) + 1 RV₁ & I_(M) ¹/M_(M)¹ . . . . . . N · M RV_(N) & I_(M) ^(N)/M_(M) ^(N) . . . . . .

[0092] As shown in the example, the mappers/interleavers I_(m)^(n)/M_(m) _(n) could be the same for all redundancy versions n, i.e.mappers/interleavers are independent from n: I_(m)/M_(m) (in total Mdifferent mappers/interleavers). The mapping rules or interleaverpatterns might be chosen such that the averaging process for both thesystematic bits and parity bits is as good as possible. Any pair ofmappers/interleavers I_(m) _(n)/M_(m) ^(n), I_(k) ^(j)/M_(k) ^(j) mighthave the same mapping rule or interleaver pattern.

[0093] Preferably, the number M of mappers/interleavers might be chosenaccording to the number of bit-reliability levels caused by themodulation scheme. Alternatively, the number M of mappers/interleaversmight be chosen according to the twice the number of bit-reliabilitylevels caused by the modulation scheme.

[0094]FIG. 11 shows an exemplary first embodiment of a communicationsystem in which the method underlying the invention is employed.

[0095] At the transmitter 100, a bit sequence is obtained from a forwarderror correction (FEC) encoder (not shown) and subsequently input intoan interleaver 110 and a logical bit inverter 120. The interleaver 110and logical bit inverter 120 are each functions of the redundancyversion and/or the mapper/interleaver version m and modify the input bitsequence. Subsequently, the bit sequence is input into themapper/modulator 130 being the mapping entity. The mapper typically usesone of the signal constellations shown in FIG. 2 and maps the bits ontoa symbol which is transmitted over the communication channel 200. Thecommunication channel is typically a radio communication channelexperiencing unreliable and time-varying channel conditions.

[0096] The patterns used by the mappers, interleavers and inverters areeither stored at both, the transmitter and the receiver or stored at thetransmitter and signalled to the receiver.

[0097] At the receiver 300, the complex symbols are first input into ade-mapper/demodulator 330 which demodulates the received symbols into acorresponding bit domain sequence (e.g. sequence of LLRs). This sequenceis then input into a logical inverter 320 and subsequently into ade-interleaver 310 from which the obtained bit domain sequence isoutput.

[0098] The interleaver and de-interleaver operate in accordance with thewell known technique of interleaving/deinterleaving by applying adetermined, pseudo-random or random permutation of the input bit orsymbol sequences, i.e. change the positions of the bits or symbolswithin a sequence. In the above described embodiment, the interleaver(and the deinterleaver) are a intra-symbol bit (de-)interleaver whichchange the position of the bits that form a symbol in themapper/demapper.

[0099] The logical bit inverter operates in accordance with a well knowntechnique of inverting the logical value of a bit, i.e. turns a logicallow to a logical high value and vice versa. In one practical realizationof a receiver working with log likelihood ratios, this invertingoperation is equivalent to a sign inversion of the log likelihood ratio.

[0100] If a retransmission is launched by an automatic repeat requestissued by an error detector (not shown) with the result that anotherdata packet is transmitted from the transmitter 100, in thede-mapper/demodulator 330, the previously received erroneous datapackets are combined with the retransmitted data packets. Due to themodification of the bit sequence by the interleaver and the logical bitinverter, the mean bit reliabilities are averaged out resulting in anincreased performance in the receiver.

[0101] As an alternative approach, in the second embodiment shown inFIG. 12, the pattern for interleaving/de-interleaving the bit sequencebefore sending same to the mapper is left constant to i.e. does notchange as a function of the redundancy version n. Instead, the rules formapping the bits onto a symbol are changed which corresponds to havinginput bit sequences into the mapper only depending on the redundancyversion n and simply changing the bit-to-symbol mapping rules.

[0102] In a further variant, not explicitly shown in the figures, acombination of the two described approaches above can be used, i.e.mapper/interleaver and inverter depend on the redundancy version n andthe mapper/interleaver version m.

[0103]FIG. 13 shows the result of a simulation measuring the frame errorrate for a 16-QAM modulation scheme employing a code rate 1/2 for twoconventional HARQ methods and one possible implementation of the methodaccording the present invention. For this example, strategy 2 in belowtable 5 has been compared with two conventional strategies. It isobvious from FIG. 13 that the method according to the inventionoutperforms the conventional methods. TABLE 5 Conventional 1Conventional 2 (using identical (alternating between Strategy 2 Schememapping for mappings irrespective (according to Transmission alltransmissions) of redundancy version) Table 3) 1. TX RV₁ & Mapping 1 RV₁& M¹ RV₁ & M¹ (M₁) 2. TX RV₂ & M¹ RV₂ & M² RV₂ & M² 3. TX RV₁ & M¹ RV₁ &M¹ RV₁ & M² 4. TX RV₂ & M¹ RV₂ & M² RV₂ & M¹

[0104] In the table, the used redundancy versions (RV_(n)) and mappings(M^(m)) for simulated methods are listed, where the mappings M₁ ¹=M₂¹=M¹ and M₁ ²=M₂ ²=M² are according to Table 4 (i.e. identical mappingsused for both redundancy versions). M¹ corresponds to constellation 1and M² corresponds to constellation 2 in FIG. 2.

[0105] Although the method described above has been described usingGray-encoded signals and a QAM modulation scheme, it is clear to askilled person that other suitable encoding and modulation schemes, e.g.PSK-modulation can be equally used in obtaining the benefits of theinvention.

1. An ARQ retransmission method in a communication system, wherein datapackets comprising modulation symbols are retransmitted based on anautomatic repeat request and subsequently combined with previouslyreceived data packets, comprising the steps of: modulating the symbolsof said data packets employing a predetermined signal constellation;retransmitting the data packets in the form of a selected one of aplurality of different redundancy versions; and reordering the bits tobe transmitted prior to modulation over the retransmissions inaccordance with the selected redundancy version.
 2. The retransmissionmethod according to claim 1, wherein the step of reordering comprises adifferent signal constellation selected cut of a plurality of signalconstellations in accordance with the selected redundancy version. 3.The retransmission method according to claim 1, wherein the step ofreordering comprises interleaving the bits or symbols of the datapackets prior to modulation according to a selected one out of aplurality of different interleaving patterns.
 4. The retransmissionmethod according to claim 1, wherein the step of re-ordering compriseslogical bit inversion according to a selected one of a plurality ofdifferent inversion patterns.
 5. The retransmission method according toclaim 1, wherein the number of signal constellations, interleavingpatterns and/or inversions patterns is according to the number or twicethe number of bit reliability levels caused by the signal constellationemployed as modulation scheme.
 6. The retransmission method according toclaim 1, wherein the modulation scheme employed is quadrature amplitudemodulation (QAM) of a higher order wherein more than two bits are mappedonto one symbol.
 7. The retransmission method according to claim 1,wherein the symbol bits of the data packets are Gray encoded.
 8. Theretransmission method according to claim 1, wherein the modulationscheme employed is 16 QAM and that during modulation, one of two levelsof mean bit reliability are assigned to each of the four symbol bits. 9.The retransmission method according to claim 1, wherein modulationscheme employed is 64 QAM and that during modulation, one of threelevels of mean bit reliabilities are assigned to each of the six symbolbits.
 10. The retransmission method according to claim 1, wherein apattern which determines the interleaving and inverting process isstored at the receiver and the transmitter of the communication system.11. The retransmission method according to claim 1, wherein thetransmitted bits comprise systematic and parity bits and that thesystematic bits are included in each redundancy version.
 12. Theretransmission method according to claim 1, wherein the transmitted bitscomprise systematic and parity bits and that the parity bits areincluded in each redundancy version.
 13. The retransmission methodaccording to claim 1, wherein each symbol bit has a mean bit reliabilitydefined by the individual bit reliability level over all symbols of thepredetermined signal constellation
 14. The retransmission methodaccording to claim 13, wherein the mean bit reliabilities for thesystematic bits are higher than that of the parity bits.
 15. Theretransmission method according to claim 14, wherein the reordering ofthe bits to be transmitted over the retransmissions is such that thecombined mean bit reliabilities for the respective bits are averagedout.
 16. A transmitter in an ARQ communications system, wherein datapackets comprising modulation symbols are retransmitted based on anautomatic repeat request, comprising: a mapping unit for modulating thesymbols of said data packets, employing a predetermined signalconstellation; a transmission unit for retransmitting data packets inform of a selected one of a plurality of different redundancy versions;and means for reordering the bits to be transmitted prior to inputtingsame into the mapping unit over the retransmissions in accordance withthe selected redundancy version.
 17. The transmitter according to claim16, further comprising an interleaves and a logical bit inverter adaptedto use a pattern which determines the interleaving and invertingdepending on the selected redundancy version.
 18. The transmitteraccording to claim 16, wherein the interleaver is an intra-symbolinterleaver.
 19. A receiver in an ARQ communication system, wherein datapackets comprising modulation symbols are retransmitted based on anautomatic repeat request and subsequently combined with previouslyreceived data packets, comprising: a demapping unit for demodulating thesymbols of said data packets employing a predetermined signalconstellation; means for receiving data packets being retransmitted inthe form of a selected one of a plurality of different redundancyversions; and means for reordering the bits of the received modulationsymbols after demodulation over the retransmissions in accordance withthe selected redundancy version.
 20. The receiver according to claim 19,further comprising a de-interleaver and a logical bit inverter adaptedto use a pattern which determines the interleaving and invertingdepending on the selected redundancy version.
 21. The receiver accordingto claim 20, wherein the de-interleaver is an intra-symbolde-interleaver.